Method of exploration for hydrocarbons

ABSTRACT

A method of exploring for hydrocarbons is disclosed wherein core is obtained over substantially the entire length of a well and substantially all lithologies represented within the core are analyzed at a core analysis facility located adjacent the well. A measurement of a plurality of physical properties of the core are made promptly after the core is removed from the well. Thereafter, from the measurement of these physical properties, a depth correlated record is generated, and is used (a) to adjust the drilling plan for that well while it is being drilled, (b) used as inputs into other exploration processes, such as seismic surveys, magnetic surveys and gravity surveys, or (c) used for correlation with other depth correlated records obtained from other wells.

This is a continuation of copending application Ser. No. 265,073, filedOct. 21, 1988, now U.S. Pat. No. 5,012,674.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the exploration for hydrocarbons and,more particularly, to obtaining core from a wellbore and promptlyanalyzing the core at the well site to develop a better understanding ofthe earth's subsurface.

2. Description of the Prior Art

In the exploration for hydrocarbons, the search is concentrated onlocating sedimentary rocks which have produced, contained or trappedhydrocarbons. To this end, first of all an understanding of the earth'ssubsurface is developed for a particular area of interest, usually fromcommercially available large area survey services. Such surveys caninclude gravity surveys, magnetic surveys and seismic surveys. Gravityand magnetic surveys are attractive because one can obtain large arealcoverage for relatively minimal cost.

Problems with gravity and magnetic surveys are that each suffers fromnonuniqueness, lack of resolution and each only provides an estimate ofthe relative thickness of material that lies above a layer of materialwith a higher density or magnetic response. Further, these surveysprovide little information as to what type of material is below thesubsurface or what is below the layer of material with the higherdensity or magnetic response. A problem with seismic surveys is that thetraveltime between source initiation, reflection and receipt is knownbut the depth to the reflecting points is only an estimate based upon anassumed velocity that the seismic energy travels through each layer ofthe earth's subsurface. If any one velocity estimate is incorrect, thenthe depth estimates and the understanding of the subsurface can begreatly altered.

If the first understanding of the earth's subsurface indicates possiblesediments, other sources of information are used to refine thisunderstanding, such as geological descriptions of surface rocks and rockoutcrops and subsurface information from any adjacent mines and/orpreviously drilled wellbores. From this refined understanding, a welllocation is made and drilling of the well is commenced. During and afterthe well has been drilled, a series of wireline wellbore logs areusually obtained, such as gamma ray, pulsed neutron and resistivitylogs, that are used to estimate the mineralogy of the subterraneanformations, the presence of hydrocarbons, and inferred rock properties,such as permeability and porosity.

It is recognized that the desired measurement of the physical propertiesof the earth's subsurface can be best understood by analyzing actualrock samples of the earth's subsurface formations. Such rock samples canbe obtained through commercially-available coring services. One of suchcoring service that has been used extensively for mineral explorationand occasionally for hydrocarbon exploration is described in "ContinuousWireline Core Drilling: An Alternative Method for Oil and GasExploration" R. E. Swayne, Drill Bits, Spring 1988 drill bits, pages7-12. This article also mentions that direct correlations can be made bycomparing wireline well logs obtained from the well to measurements madeon core samples over the entire length of the well.

Several uses of information obtained from core analysis are described in"Reservoir Description: What Is Needed and When?" by Richardson, et al.,published in Symposium on Geology and Reservoir Management, 1986,National Conference on Earth Science, September 1986.

SUMMARY OF THE INVENTION

The present invention provides an exploration method and system thatassists an explorationist in obtaining a better understanding of theearth's subsurface. In one embodiment of this invention, core isobtained over substantially the entire length of a wellbore andsubstantially all the lithologies represented within the core areanalyzed within a core analysis facility located adjacent the well toobtain a better understanding of physical properties of the earth'ssubsurface. Thereafter, a representation, such as a depth correlatedrecord or log, can be generated within the core analysis facility of theearth's subsurface that can be used in making decisions during thedrilling of that well and later for exploration and exploitationpurposes.

Once a core is removed from the well, connate fluids rapidly evaporate,certain fragile formation materials quickly disintegrate, and corerelaxes viscoelastically. The inventors hereof have found that certainof these physical properties change more rapidly than previouslythought. By promptly analyzing the core at the well, the core can beanalyzed within an acceptably short period of time, with minimaldisturbance to the core, and under conditions such that its physicalproperties will not be substantially different from those of the corewhen first removed from the well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of core being withdrawn from awell and being transported to a core analysis facility in accordancewith one embodiment of the present invention.

FIG. 2 is a diagrammatic elevational view of a continuous core samplingstation within a core analysis facility, integrated as part of thepresent invention.

FIG. 3 is a depth correlated log output from one embodiment of a coreanalysis facility.

FIG. 4 is a diagrammatic view of a discrete core sampling station withina core analysis facility, integrated as part of the present invention.

FIG. 5 is a depth correlated ultraviolet log taken at two differenttimes over the same core showing change in the measured ultravioletresponse with the passage of time.

FIG. 6 is a graph of the percent change in measured velocity with thepassage of time.

FIG. 7 is a diagrammatic representation of correlating a log obtainedfrom core analysis of the present invention to a conventional logobtained from an offset well.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with one method of the present invention, core is takenfrom a well, promptly transported to a core analysis facility adjacentthe well, and then substantially all lithologies represented within thecore are promptly analyzed to obtain accurate measurements of aplurality of physical properties. These measurements can then be used byan explorationist and/or production personnel to generate a moredetailed and accurate understanding of the earth's subsurface, such asby viewing a depth correlated record or log of the measured physicalproperties.

The method of the present invention involves at least two majorcomponents: Drilling/Coring and Core Analysis; both of which will bedescribed below.

Drilling/Coring.

Any commercially available drilling and coring rig and associatedequipment and supplies can be utilized within the practice of themethods of the present invention. Commercially available rotary coringtechniques and wireline provided sidewall coring can be utilized. Aparticular type of core drilling rig and associated equipment that hasbeen successfully utilized in the practice of this embodiment of thepresent invention is described in "Continuous Wireline Core Drilling,"Swayne, p. 7. High speed, small diameter, low weight-on-bit coring ofthe type described in the Swayne article is preferred because of anumber of reasons. The small diameter, usually less than about 6 in.,wellbore produced by this type of drilling does not encounter theexpensive and troublesome problems of wellbore stability that is usuallyexperienced with larger diameter wellbores. There is less of a tendencyfor smaller diameter wellbores to become out of gauge (too large) or forformation material to sluff into the wellbore because with a smallerdiameter wellbore, the integrity of the formation is preserved. With theflush-type drillstring used, a small annulus results between thewellbore face and the drillstring. Thus, less drilling fluid is requiredand less pumping capacity is needed than that usually needed with largerdiameter wellbores. Therefore, a more expensive drilling fluid that willcause less damage to the core and formation adjacent the wellbore can beused throughout the entire coring/drilling operation. Further, thecoring drillstring usually rotates at greater than about 400 rpm whichprovides higher penetration rates and does not damage the formationmaterial as much as large diameter core bits rotating at lower rates.

Core Analysis.

As shown in FIG. 1, in the practice of one embodiment of the presentinvention, lengths of core, usually 20' or 40', are withdrawn from thewellbore and each length is transported to an adjacent core analysisfacility. The core is placed on a carrying rack and passed through aliquid (such as water) bath, wash or spray to remove any drilling fluidor other foreign material from the surface of the core and then passedthrough an air spray to remove the wash liquid from the core. The washof liquid and air spray are not mandatory but are preferred to removeany material that could adversely affect a measurement of a particularphysical property. Each length of core is cut into appropriate sizes foranalysis and storage, such as 3' lengths. Discrete samples are cut fromthe core for physical property analysis within a discrete sampleanalysis station, as will be described later.

The core analysis methodology of the present invention can be dividedinto two distinct processes: continuous core sample analysis anddiscrete core sample analysis. In one embodiment of the continuous coresample analysis methodology shown in FIG. 2, after the core has been cutinto convenient lengths for analysis and handling, a bar code label isattached to each length for inventory control and then each length ispassed through one or more physical property measurement devices toprovide the indication of the physical properties of the core. Thephysical properties that can be measured within this station comprisegamma ray emission which is useful for correlating zones with gamma raylogs from other wells, magnetic susceptibility which is useful forproducing a representation of the earth's magnetic characteristics,infrared reflectance which is useful for determining mineralogy, andultraviolet fluorescence which is useful for detection of hydrocarbons.A visual record of the core is made, such as on a photograph, videotapeor laser disk, to record a macroview, such as about 3 in. square visualsample, and a microview of a portion of the macroview, such as by a 10power microscope.

The length of core is then passed to a zone where depth correlated corelog, record or description is made, usually by a geologist, to recordthe general lithology type, i.e., carbonate, sandstone, shale, etc., andgeological characteristics, such as bedding planes, faulting, diporientation, depositional environment, depositional history, tectonics,fossil description and the like. An example of a depth correlated logoutput from such a core analysis facility is shown in FIG. 3, wherein adepth correlated record or log is made in analog form of gamma-ray andultraviolet fluorescence, a pictorial representation is made oflithology, and estimated measurements are made of carbonate content andporosity. Thereafter, the core can be discarded, boxed and stored onsite or all or portions of the core can be transported to a coreanalysis facility for later analysis and use.

In the discrete core sample analysis methodology, discrete samples ordiscs are taken from the core at predetermined sampling intervals. Asampling interval is at least every lithology change. The intervalshould be sufficient to adequately represent the physical properties forthe wavelength of maximum resolution used for seismic processing, and nogreater than every 10'. In one embodiment of the present invention,shown in FIG. 4, at least three disc samples are taken for eachlithology and are tested within testing equipment to obtain one or moreof the following physical properties: grain density, dry bulk density,saturated bulk density, magnetic susceptibility, mineralogy (infrared),compressive strength, elastic moduli and compressional and shear seismicvelocities versus pressure. Having these direct measurements one cancalculate other physical parameters of interest. For example, thedifference between the dry bulk density and the grain density yields aporosity estimate. Likewise, the difference between the saturated anddry bulk densities yields another estimate of porosity. Further, knowncombinations of saturated density and seismic velocities yield dynamicelastic moduli, acoustic impedance and reflection coefficients.Piecewise integration of velocity over the depth interval sampled yieldsan estimate of the two-way traveltime, which can be used to convert thetime observed on a seismic section to a true depth. The above describedmeasurements also permit a measurement of P and S wave velocities,birefringence, the magnitude of the difference between the fast and slowshear velocities, and elastic fabric angle. If these measurements arecoupled with the sample's measurements (diameter, weight, and length),one can estimate porosity. Also, the difference between saturated anddry density provide a saturated porosity estimate. A depth correlatednumerical log of measured properties from the above described discretesampling facility of FIG. 4 is shown in Table I.

                                      TABLE I                                     __________________________________________________________________________         DENSITIES                                                                            POROSITIES                                                                            VELOCITIES                                                                             MAG DIMEN                                        DEPTH                                                                              GM/CM 3                                                                              %       FT/SEC   Cgs/                                                                              mm                                           FEET Grain                                                                             Sat                                                                              Pycn                                                                              Sat Vp Vs11                                                                             Vs22                                                                             gm X                                                                              dia                                                                              len                                       __________________________________________________________________________    301.0                                                                              2.604                                                                             2.182                                                                            24.2                                                                              20.8                                                                              8056                                                                             3868                                                                             4195                                                                             -.01                                                                              63.1                                                                             38.2                                      312.0                                                                              2.610                                                                             2.207                                                                            24.3                                                                              23.1                                                                              8130                                                                             4693                                                                             3850                                                                             .07 62.9                                                                             37.9                                      321.0                                                                              2.635                                                                             2.197                                                                            25.8                                                                              24.3                                                                              8444                                                                             4022                                                                             4054                                                                             .14 61.9                                                                             38.0                                      340.0                                                                              2.626                                                                             2.232                                                                            23.5                                                                              22.3                                                                              8115                                                                             3649                                                                             3581                                                                             -.53                                                                              63.3                                                                             38.4                                      352.0                                                                              2.635                                                                             2.232                                                                            23.0                                                                              20.2                                                                              8097                                                                             5532                                                                               0                                                                              .25 62.9                                                                             38.2                                      365.0                                                                              2.673                                                                             2.192                                                                            26.3                                                                              22.2                                                                              7683                                                                             5388                                                                             4114                                                                             .12 63.3                                                                             38.1                                      375.0                                                                              2.621                                                                             2.192                                                                            22.0                                                                              14.8                                                                              7752                                                                             4647                                                                             3871                                                                             .26 63.2                                                                             38.2                                      385.0                                                                              2.633                                                                             2.192                                                                            24.2                                                                              19.6                                                                              8351                                                                             3927                                                                             3960                                                                             .22 63.4                                                                             38.5                                      395.0                                                                              2.615                                                                             2.156                                                                            25.6                                                                              21.0                                                                              8686                                                                             4321                                                                             4262                                                                             .22 63.1                                                                             38.2                                      405.0                                                                              2.621                                                                             2.172                                                                            26.6                                                                              24.8                                                                              8274                                                                             4209                                                                               0                                                                              .27 63.5                                                                             38.2                                      __________________________________________________________________________

Preferable Analysis Methods

Some or all of the measured physical properties from the continuous andthe discrete sampling portions of the core analysis facility can berecorded in hardcopy form, visual form or within a memory associatedwith a digital computer. These physical property measurements can beprovided to locations remote from the well site by telephone, fax link,digital communication link and the like. In one embodiment, every daythat a well is being drilled, a predetermined subset of the measuredphysical property data is transmitted via a satellite link to a databasewithin a remote computing facility for an explorationist to develop andrefine his/her understanding of the earth's subsurface for that well.Also, other explorationists can access the database for use in improvingtheir understanding of the earth's subsurface at adjacent well sites orwell sites completely removed and distinct therefrom. In other words,the measured physical properties can be added into a database which canbe used by explorationists to obtain a constantly updated and improvingunderstanding of subsurface phenomena from around the world.

In the analysis of the core, the sample interval has been found to bevery important because as more lithologies are not analyzed theinterpretations of the data become more generalized and thus moreuncertain. If core from a certain zone is analyzed and the otherportions of the wellbore are not cored or the core is not analyzed thenneeded information will be lost. Further, substantially all lithologieswithin a core need to be analyzed because each lithology affects thetime acoustic energy travels through the earth's subsurface.Specifically, if a surface formation's velocity is not measured, thensuch subsurface formation's impedance must be estimated for use inseismic processing. If the estimate is incorrect, then such error cancause the depth to subsurface reflectors to be in error.

Certain physical properties of the core have been found to change muchmore rapidly and detrimentally than previously thought with exposure todecompressional, drying and oxidizing conditions and with the passage oftime. It is a primary goal of the present invention to promptly analyzethe core to obtain a measurement of physical properties approximatelyequivalent to the physical properties existing at the time of removingthe core from the well.

For example, hydrocarbons can evaporate quickly so that an ultravioletfluorescence log taken immediately after the core is removed from awellbore can show hydrocarbons, yet the same log run as little as threehours later will shown no such presence of hydrocarbons. For an exampleof this, please see FIG. 5. Since in the use of the present invention,one is trying to explore for and hopefully find hydrocarbons, a showingof an oil film on a core at a depth correlated zone can be ofsignificant importance to refining an explorationist's understanding ofwhether or not hydrocarbons are present, and if so, where did thehydrocarbons come from and where additional hydrocarbons may be. Withthis information, an explorationist can then make decisions as towhether or not drilling should continue, if so, how much further, and ifanother well is planned/needed, where to locate the new well. In thepast, core taken from a well was transported to a core analysis facilitythat was remote from the well, and the core analysis usually doneseveral weeks to months later. Thus, valuable data was lost. Yet, withthe use of present invention, this valuable data can be generated andused when needed.

Other properties negatively affected by the passage of time betweenwellbore removal and analysis are hydrocarbon pyrolysis and pore fluidanalysis, compressive strength, and compressional and shear velocities.FIG. 6 shows a graph of a percent change in compressional and shearvelocities on a depth correlated scale with the passage of four (4)months between the measurements taken promptly after the core is removedfrom the well and subsequent measurements taken. These velocity changesare significant because incorrect velocity estimates negatively affectdepth estimates to reflectors. For example, a depth estimation made froma seismic plot using a velocity that is 10% slower than the actualvelocity yields a time to an event at 10,000' of 2.2 seconds rather than2.0 seconds. An error of 200 milliseconds in projected additionalpenetration of about 1000' to 1500'. Such errors negatively impact theeconomics of finding a desired zone and the certainty of attaining thedrilling objectives.

Further, velocity estimates for shallower depths are nonexistent or, atbest, poorly constrained. For example, a shallow high velocity zone wascored and the measured velocities for the zone proved to be markedlyfaster than the estimated velocities used in processing the seismicsection. The original estimates led to a predrilling depth estimate ofthe objective zone of 7900'. By using the actual measured velocities ofthe core obtained from the core analysis facility adjacent the well, theseismic section was reprocessed and the objective zone was estimated tobe at 6000'. The drill bit later cored the objective zone at 6030'.

This use of the velocity data permits one to stop drilling if desired toforego the cost of having to drill to the originally planned 7900'.Further, usually a vertical seismic profile would have been obtainedusing a wireline logging tool to confirm the penetration of theobjective zone; however, with the confidence gained from the on-sitecore analysis, the operator was able to forego this additional expense.As an added point, similar drilling estimates to objective zones can bedone using velocity logs; however, such logging is not preferred,compared to the on-site core analysis of the present invention, becauseto use such logging methods the drilling must be stopped, marginallystable wellbores can be damaged and collapse, and additional time andmoney is needed to acquire and process these logs. The present inventionprovides a method to obtain needed physical property measurements fromcore as an integral part of the drilling process.

Certain formation materials, such as shales, disintegrate very quicklywith exposure to air, vibration and release from their in-situconditions. Usually, shales are not analyzed at a remote core analysisfacility because the shales have disintegrated; thereby valuableinformation is lost. Therefore, it is imperative to analyze theseformation materials on-site if one hopes to obtain any meaningfulmeasurements of their physical properties and description of theircharacteristics.

Use of the Measured Properties

The following discussion is provided to permit a better understanding ofadditional uses of the information generated within the core analysisfacilities, in accordance with the present invention.

As described above, the physical properties of the core material can beused at the well site to recalculate the depth of reflectors, i.e.,horizons to be drilled, and the distance to target or total depth (TD)of the well. In the past, the production personnel drilling the well hada rough guess of the depth to which they are to drill to reach aparticular desired horizon. However, that horizon is almost alwaysestimated from offset well logs and/or seismic data, which in turn isheavily dependent upon assumed rock properties and velocities. With thetimely core analysis provided by the preset invention, the drillingpersonnel can determine whether or not they have passed through aparticular horizon of interest and at what depth a particular horizonwas passed through. Also, a prediction can be timely made of how muchlonger a particular formation is to be drilled before a formation changeis expected, so the drilling personnel can determine whether or not tomake a bit change and/or a fluid change and when to make such change(s).

Drill bit performance can be determined by knowing the lithology andcompressive strength of the formation from the timely core analysis andknowing the rate of penetration of that bit through the formation, thusone can monitor the degradation of bit performance caused by bit wearand/or bit failure. Also, one can establish an understanding of how wella particular drill bit drills through a particular formation material.In the past, these types of accurate determinations could not be madebecause prompt, on-site instrumental core analysis was not available.

Because of timely core analysis, information obtained from the well canbe quickly utilized for development or refinement of additionalexploitation, exploration, well drilling, and well completion plans forother wells in the same or other areas. The density data and magneticsusceptibility data can be used to reprocess gravity and magneticsurveys. Timely analysis can shorten play evaluation time which leads toa higher efficiency by exploration personnel. Thus, rapid determinationscan be made of whether or not to bid on a concession/lease, how much tobid, if and where additional wells are to be drilled within theconcession/lease and the like.

A form of logging, called inverse logging, can be accomplished wherein awell is drilled, core obtained therefrom and analyzed in accordance withthe present invention. Thereafter, the depth correlated log of the corecan be correlated to other logs, such as a gamma ray log, obtained fromoffset wells to establish the location and depth of the same zonespenetrated by both wellbores. Further, from the logs generated by way ofthe present invention, the lithology and other physical properties canbe reasonably inferred at the offset well(s). An illustration of thistechnique is diagrammatically shown in FIG. 7.

By having substantial quantities of core, from a well, such as core fromsubstantially the entire length of the well and having that coreanalyzed on-site, more of the entire picture of the depositionalenvironment of the earth's subsurface can be determined. Thedepositional environment will include whether or not there were beaches,streams, dunes, and the like present, and have any alterations takenplace. The tectonic history can be determined as well as paleontologicalsequencing. The diagenetic history can be determined as well as thetypes and distribution of fossils. Depositional environment, rockfabrics, diagenetics, burial history, the presence of source, reservoirand cap rocks, can also be determined. Reservoir quality andcharacteristics, rock type and distribution, can all be determineddirectly from the core on site in such a manner that these propertiescan provide information directly to the explorationist to better refineand evaluate the subsurface model. The porosity, fracture concentrationand orientation can provide indications of the areal and verticalcontinuity of a basin.

Future stimulation procedures for the cored well, such as fracturing,acidizing and the like, can be more accurately designed because actualcore is analyzed to obtain the necessary inputs, such as formationparting pressures, permeability, Poisson's ratio and Young's moduli andthe like.

Wherein the present invention has been described in particular relationto the drawings attached hereto, it should be understood that other andfurther modifications, apart from those shown or suggested herein, maybe made within the scope and spirit of the present invention.

What is claimed is:
 1. A method of producing a depth correlated recordof the earth's subsurface using core cut as a well is being drilled,comprising:(a) within a core analysis facility adjacent the well,analyzing substantially all lithologies represented within the core andobtaining a measurement of at least one physical property thereof, thephysical property being a time-dependent physical property affected bypassage of time between core removal from the well and making of themeasurement, the measurement being obtained promptly after the core isremoved from the well and under conditions including time effective forthe at least one physical property being substantially equivalent to thephysical property existing in the wellbore at time of removing the core;and (b) producing a depth correlated record of the at least one physicalproperty.
 2. The method of claim 1 wherein the physical property isselected from the group consisting of shear acoustic velocity,compressional acoustic velocity, and compressive strength.
 3. The methodof claim 1 wherein core is obtained from substantially the entire lengthof the well and is provided to the core analysis facility.
 4. The methodof claim 1 wherein the depth correlated record of at least one physicalproperty is correlated to a depth correlated log obtained from anotherwell.
 5. A depth correlated record of the physical properties made inaccordance with claim
 1. 6. A core analysis facility for use adjacent awell to produce a depth correlated record of the earth's subsurface,comprising:means for providing core having physical propertiesapproximately equivalent to physical properties existing at time ofremoving core from a well; analysis means adjacent the means forproviding core for analyzing substantially all lithologies representedwithin a core removed from the well under conditions including timeeffective to obtain a measurement of at least one physical propertythereof, the physical property being a physical property affected bypassage of time between core removal from the well and making of themeasurement; and means for producing a depth correlated record of the atleast one physical property.
 7. The facility of claim 6 wherein themeans for providing core comprises:drilling and coring means fordrilling a well and removing core therefrom.
 8. The facility of claim 6wherein the physical property is selected from the group consisting ofultraviolet fluorescence, hydrocarbon pyrolysis, pore fluid analysis,shear acoustic velocity, compressional acoustic velocity, andcompressive strength.
 9. A method a revised drilling plan used inexploring for hydrocarbons, comprising:(a) developing a representationof the earth's subsurface for an area; (b) developing a drilling planfor the area from the representation of the earth's subsurface; (c)drilling a wellbore to penetrate the earth's subsurface in accordancewith the drilling plan; (d) obtaining a core over substantially theentire length of the wellbore; (e) analyzing substantially alllithologies represented within the core at a location adjacent thewellbore to obtain a plurality of physical properties thereof promptlyafter the core is removed from the wellbore; (f) revising therepresentation of the earth's subsurface from the obtained plurality ofphysical properties; (g) revising the drilling plan for the area inresponse to the revisions in the representation of the physicalproperties of the earth's subsurface; and (h) drilling additionalwellbores in the area in accordance with the revised drilling plan. 10.The method of claim 9 wherein the plurality of properties includecompressive strength and porosity.
 11. The method of claim 9 wherein therepresentation of the earth's subsurface comprises a seismic section.12. A method of exploring for hydrocarbons utilizing a comparison ofdepth correlated records from a first and a second well comprising:(a)obtaining a depth correlated record of at least one physical property ofa core taken from substantially the entire length of a wellbore analyzedat a core analysis facility adjacent a first well from which the corewas obtained and analyzed promptly after the core is removed from thefirst well; (b) obtaining a depth correlated record of the at least onephysical property from a wireline log passed through a second well; and(c) comparing similar indicated subsurface features from the record ofstep (a) to the record of step (b).
 13. The method of claim 12 whereinthe at least one physical property is natural gamma ray radiation. 14.The method of claim 12 and including:(d) developing an inferredlithology log for the second well from the depth correlated record of atleast one physical property of the first well.